this will allow an airspeed sensor object to be passed to AHRS for dead reckoning
this also adds a healthy attribute, and error checking on all I2C calls
Most of the compass functionality is now abstracted in a base class, with the various sub-classes implementing just their unique functionality. git-svn-id: https://arducopter.googlecode.com/svn/trunk@1647 f9c3cf11-9bcb-44bc-f272-b75c42450872
Andrew Tridgell
Pat Hickey
DrZiplok@gmail.com